1. Field of the Invention
The present invention relates to hydraulic suspension systems for mitigating transmission of vibrations from one section of a motor vehicle to another section, such as transfer of wheel or axle motion to the vehicle chassis; and more particularly to suspension systems that employ a double acting hydraulic cylinder and an accumulator.
2. Description of the Related Art
A motor vehicle suspension system includes at last one hydraulic suspension cylinder connected between the vehicle chassis and a wheel and/or vehicle axle. Simple suspension systems employ conventional a shock absorber which comprises a sealed cylinder having an internal piston with an orifice that controls flow of fluid between piston and rod chambers on opposite sides of the piston. The constrained rate of fluid flow governs piston movement, thereby damping motion between the chassis and the wheel and/or axle.
More sophisticated suspension systems are adjustable wherein the damping characteristic which can be varied dynamically in response to changes in the load on the vehicle. In one system of this type, a gas-charged accumulator pressurizes at least one of the cylinder chambers which allows resilient displacement of the piston. A valve arrangement selectively connects the piston chamber and rod chamber to a pressure source and a tank of the hydraulic system.
U.S. Pat. No. 6,394,238 describes a suspension system that operates in a regeneration mode, in which a regulated fluid path is provided between the piston and rod chambers. This system includes an accumulator that stores the relatively small amount of fluid corresponding to the difference in volume between the rod and piston chambers, i.e. the portion that the piston rod occupies in the rod chamber. The regeneration mode is particularly useful on vehicles that handle a relatively small range of loads. However, this type of suspension has limited application for large trucks and equipment used in agriculture and construction, because of the wide ranging vehicle loads that will be encountered. In addition, the typical regeneration system cannot actively drive the suspension downward, thereby relying on gravity for this function.
U.S. Pat. No. 6,575,484 describes a suspension system that operates in both a regeneration mode and a double acting mode, in which the cylinder chambers are isolated from each other and fluid flows between them and separate accumulators. The selection between those modes is made in response to the pressure level in the cylinder as measured by a pressure sensor of a closed loop control system. A double-acting cylinder with the rod and piston chambers connected to separate accumulators has been used on vehicles that handle a wide range of loads. However, the accumulators for a double acting system must accommodate the entire volume of the respective cylinder chamber, thus being significantly larger than the accumulator in a regeneration system that merely has to accommodate the volume of the rod within the rod chamber.
Because accumulator size affects the cost and space required, it is desirable to reduce the size of the accumulators in a double-acting suspension system.